This invention relates generally to sol-gel processes for producing dry gel monoliths that subsequently can be sintered into metal oxide articles and, more particularly, relates to sol-gel processes of this kind using molds specially configured to enhance the process"" effectiveness.
Substantial efforts have recently been expended in developing improved sol-gel processes for fabricating high-purity monolithic articles of metal oxide. In such processes, a desired solution, i.e., a sol, containing metal-oxide-forming compounds, solvents, and catalysts, is poured into a mold and allowed to react. The solution typically includes a metal alkoxide, water, an alcohol, and an acid and/or base catalyst. Typical metal alkoxides include tetraethyl orthosilicate (for forming articles of silica) and tetrabutyl titanate (for forming articles of titanium dioxide). Following hydrolysis and condensation reactions, the sol forms a porous matrix of solids, i.e., a gel. With aging, the gel shrinks in size by expelling fluids from the pores of the gel. The wet gel is then dried in a controlled environment, typically by removing the gel from the mold and placing it into an autoclave for subcritical or supercritical heating. The dried gel then is sintered into a solid monolith.
Advantages of the sol-gel process include chemical purity and homogeneity, flexibility in the selection of compositions, the ability to process at relatively low temperatures, and the producing of monolithic articles close to their final desired shapes, thereby minimizing finishing costs.
The efficiency of the process can be enhanced if the steps of gelling, aging and drying all are carried out within a single chamber and without the need to remove the gel from the mold. The need to remove the gel from the mold at an intermediate step of the process not only requires mechanical handling of the fragile gel and mold, but also lengthens the processing time. This is because removing the gel from the mold following the step of aging can be performed only after the gel has cooled to room temperature from its aging temperature e.g., 60xc2x0 C.
An important factor bearing on the ability to perform the entire sol-gel process without removing the gel from the mold is the nature of the material from which the mold is formed. The ideal mold material should have good release characteristics, such that the fragile monolithic gel can be removed from the mold without damage.
The mold material also should be inert to attack from chemicals used in the sol-gel process, e.g., acid catalysts such as hydrochloric acid (HCl) and base catalysts such as ammonium hydroxide (NH4OH). This requirement effectively precludes the use of molds formed of metal, because metal impurities could be leached from the mold and trapped in the gel, thus being retained in the metal oxide monolith. Metal impurities retained within a metal oxide monolith are particularly undesirable because they can lead to unacceptable material properties. For example, metal ions in fused silica photoblanks can degrade transmission of ultraviolet light by the photoblanks. Such leaching also can reduce the mold""s life span.
Noble metal molds, while nonreactive, are extremely costly. Metal molds coated with Teflon or with noble metals are somewhat less costly, but the coating is usually imperfect, with pinhole openings that allow contamination of the monolith or that lead to degradation of the coating and the mold. Glass molds coated with passivating agents for noncontamination are difficult to machine and are usually unacceptably brittle.
If the gel is to be dried while still located within the mold, the mold material must be able to withstand typical drying temperatures, e.g., 200xc2x0 C. and above. This means that the mold must not decompose at such temperatures and it should not deform when repeatedly cycled between room temperature and the maximum drying temperature. This requirement effectively excludes the use of molds formed of common organic polymeric materials such as polymethyl pentane and Teflon, which have softening temperatures substantially lower than 200xc2x0 C.
It should therefore be appreciated that there is a need for a sol-gel process in which the steps of gelling, aging and drying all are carried out without removing the material from the mold. The present invention fulfills this need and provides further related advantages.
The present invention resides in an improved sol-gel process for producing a dry porous gel monolith, in which the process steps of gelling, aging and drying all are carried out while the gel remains within a mold, thus substantially reducing mechanical handling of the gel and mold and substantially enhancing the process"" efficiency. More particularly, the process incorporates steps of 1) placing a solution into a mold formed of a material such as graphite, silicon carbide, titanium carbide, or tungsten carbide, 2) allowing the solution to gel within the mold, 3) drying the gel within the mold, and 4) removing the dried gel from the mold to obtain the gel monolith.
The process is useful when used to produce gel monoliths from solutions comprising various metal alkoxides, such as SiO2, TiO2, Al2O3 and ZrO. The process has particular advantages when used to produce gel monoliths in the form of high-purity silica. In such applications, the solution consists essentially of tetraethyl orthosilicate, an alcohol, deionized water, and an acid catalyst and/or a base catalyst, in prescribed relative proportions. In addition, the process can further include a step of aging the gel within the mold, before the step of drying, and a further step of sintering the dried gel after the step of removing.
In one configuration, the mold is configured to be substantially homogeneous and porous. In an alternative configuration, the mold is configured to have a porous body with a substantially non-porous inner skin. In the alternative configuration, the non-porous skin prevents plugging of the pores of the mold with gel material, allowing for easier cleaning of the mold and reduced sticking and contamination of the metal oxide monolith. In both configurations, the pore liquid escapes from the narrow annular space between the mold and the gel. In addition, for a porous mold, the mold""s porosity facilitates this drying by allowing the liquid contained within the gel""s pores to escape directly through the mold itself. The mold preferably has a substantially uniform thickness in the range of 3 to 5 mm. In the case of molds formed of graphite, the graphite preferably has a bulk density of about 1.75 gm/cm3 and a porosity in the range of about 10 to 15%.
In other more detailed features of the invention, the steps of allowing the solution to gel, aging the gel, and drying the gel all occur while the solution and gel remain located within the mold. In addition, these steps all occur while the mold is located within an autoclave. The step of drying the gel in the autoclave can occur either under subcritical or supercritical conditions.
Other features and advantages of the present invention should become apparent from the following description of the preferred process, taken in conjunction with the accompanying drawing, which illustrates, by way of example, the principles of the invention.